High reliability is essential in instruments that are used for dimensional consistency monitoring in continuous fabrication lines—for example, for layer thickness measurement in the context of wafer manufacture in semiconductor production—since information about product quality and the stability of the production process is obtained on the basis of the measurement results. This requires that the accuracy of the measuring instrument technology used be stable.
Fundamentally, the accuracy of optical measurement arrangements depends to a considerable degree on consistent parameters of the measurement light that is generated in an illumination device. The long-term stability of the lamps used for this purpose has a great influence on the quality of the measurement arrangement. The service life of the lamps is limited, however, and as they age they become unsuitable for measurement purposes and must be replaced after a certain time. Replacement of the lamps must normally be accomplished at the site of the measurement arrangement by technicians of the manufacturer or a maintenance company, since a newly installed lamp must first be aligned with respect to the beam path in a complex operation; the reason is that a lamp that is even slightly misaligned can considerably degrade the performance of the measurement arrangement.
For the customer, this on the one hand is very time-consuming because the time period from failure of the lamp to notification of the technician (possibly not available locally) and alignment of the new lamp is in some circumstances very long, and during that period the lamp cannot be used. On the other hand, this type of lamp replacement is very cost-intensive.
U.S. Pat. No. 6,456,373 discloses an apparatus for monitoring the emitted light for such measurement arrangements, in which the lamps are automatically exchanged when the light no longer conforms to the necessary requirements in terms of measurement accuracy. This apparatus is, however, very expensive to acquire and is susceptible to failure because of its complex mechanism.
Simple apparatuses having manually replaceable lamp housings are described in the existing art, for example in U.S. Pat. No. 5,696,609 for flatbed scanners and in U.S. Pat. No. 5,860,720 for illumination of an LCD rear projector. No particularly stringent requirements in terms of illumination or alignment of the optics are imposed in these arrangements, however, since the light is not used for measurements and the arrangements thus react less sensitively to slight misalignments.
Although U.S. Pat. No. 5,696,609 indicates that provision is made for an optical alignment of the illumination module once the illumination sources have been preinstalled in the lamp housing at the factory, it leaves open the manner in which this occurs. All that is stated in the description of FIGS. 14a and 14b (columns 11-12) is that the scanner possesses an access panel. When the panel is open, the illumination module with the already-aligned illumination sources can be inserted into or removed from the scanner.
The lamp housing for illumination of an LCD rear projector described in U.S. Pat. No. 5,860,720 is inserted into an opening provided for it in the housing of the projector, and secured therein with screws. In this case as well, the illumination device is aligned with respect to the optics within predetermined tolerances, but in both cases the tolerances are much greater than would be reliable in the case of an optical measurement arrangement, where the tolerance is approximately 10 μm.